1. Field of the Invention
The invention relates to an anchoring device for releasably anchoring within a crack in a rock face, particularly adapted for, but not limited to, use by rock climbers.
2. Prior Art
There are many anchoring devices used by rock climbers to secure themselves, or equipment, in a crack of a rock face. Relatively simple anchors are shown in U.S. Pat. Nos. 3,948,485 (Chouinard et al) and 4,069,991 (Saunders et al). These devices show an irregular shaped wedge type member connected to a loop sling and adapted to be inserted within a tapering crack. The loop sling cooperates with the device in such a way as to apply a torque or twisting motion to the device when the sling is loaded. This twisting or rotation presents a device of increasing effective size within the crack, which tends to augment gripping the device as the load is increased. While this type of simple device is effective in outwardly tapered cracks, it can be difficult to remove when subjected to high loading, which could occur when a climber falls. Also, sometimes a simple wedge-type device can be accidentally pulled from cracks by the climbing rope.
The tendency of the device to present a larger size as it rotates can be referred to as "expansion ratio", which is a measure of the range of crack sizes with which the device can be used. Clearly, the greater the expansion ratio, the fewer the number of anchoring devices a climber would have to carry in order to accommodate a particular range of crack sizes. Consequently, any improvement that increases the expansion ratio of an anchoring device is a worthwhile consideration.
A more complex type of anchoring device is shown in U.S. Pat. No. 4,184,657 (Jardine) which consists of four hinged cams carried on a common spindle journalled on a rigid load bearing member. The cams are forced to rotate by springs which increase the effective size of the device, i.e., the springs expand the device to fit snugly in the crack. To remove the device from the crack, force on the springs is overcome by a retracting means. In general, these devices have a relatively wide expansion ratio, and usually they can be retrieved from the crack without much difficulty. However, when the cams are fully retracted and fitted within a crack of minimum width they can be difficult to retrieve. Also, when subjected to a rocking motion laterally within the crack, for example due to swinging of a rope, they are known to "walk" into the crack, increasing the difficulty of removal. Also, the devices are relatively complex, and have been known to fail when subjected to off-axis loads due to bending and subsequent breaking of the rigid load bearing member. European patent publication No. 0 047 232 (Meligard) discloses an anchoring device using a pair of wedges which are controlled by flexible rods, with a spring means acting between the rods to cause relative axial movement between the wedges, augmenting gripping of the device within the crack. This is a relatively simple device when compared with the previously described hinged cam member, but it is considered that it has a relatively low expansion ratio when compared with rotating cam devices. Furthermore, the wedges have a relatively large area in contact with the rock, and thus might be prone to misalignment when fitted in an irregular surfaced crack.